Conditions for maintenance of embryonic chick duodenum in organ culture in serum-free medium, responsive to vitamin D and many analogues with excellent preservation of mucosal structure, have been essentially optimized. It is proposed to further investigate the regulation of synthesis and physiological role(s) of CaBP in the calcium absorptive mechanism under the precisely controllable in vitro conditions, completely isolated from systemic influences, made possible only through the use of this unique system. Experiments to date with this technique have established that vitamin D itself (and numerous analogues) induces de novo synthesis of CaBP and stimulates transmucosal calcium transport by a calcium- dependent mechanism involving the adenyl cyclase system. Reconstitution experiments with purified CaBP have proven the involvement of this protein in the calcium transport process. Considerable further investigation of the interaction between vitamin D, CaBP, calcium and cyclic AMP will be undertaken. In this regard attempts will be made to localize and characterize hypothetical vitamin D receptors of the adenyl cyclase system. The effects of a variety of agents, known to influence calcium absorption in vivo, will be investigated. Among these are cortisone, diphenylhydantoin, EHDP and several carcinogens, the latter also to assess the possibility of using this system for studies of chemical carcinogenesis. Attempts will be made to isolate, and use in reconstitution experiments, CaBP-mRNA. Finally, through the use of series of closely related vitamin D3 analogues, a minimal structure for vitamin D activity will be investigated.